This invention relates to information-carrying medium equipment for magneto-optic reading and writing that enables direct overwriting of new information on old information.
FIGS. 5A and 5B illustrate the principle of the prior art of magneto-optic information reading and writing as shown in the Extended Abstracts (The 34th Spring Meeting, 1987); The Japan Society of Applied Physics and Related Societies, 28p-ZL-3. FIG. 5A shows an oblique view; FIG. 5B is a cross-sectional view showing the main parts. The apparatus in these drawings comprises a magneto-optic information-carrying medium 1, which in turn comprises a glass or plastic substrate 2 and two ferromagnetic layers: a first layer 3 and a second layer 4. The apparatus also comprises an objective lens 5 for focusing a laser beam onto the information-carrying medium 1, where it forms a focused spot 6. 7 designates regions in the first layer 3 in which the magnetic alignment points upward in FIG. 5B, which represents binary data "1". The apparatus also comprises two magnets: an initializing magnet 8 for creating an initial magnetic alignment in the second layer 4; and a bias magnet 9 located opposite the objective lens 5 on the other side of the information-carrying medium 1.
This apparatus operates as follows. A support and drive mechanism not shown in the drawings turns the information-carrying medium 1 in a fixed direction (indicated as direction a). The essential components of the information-carrying medium 1 are those noted previously: a glass or plastic substrate 2 and two ferromagnetic layers, a first layer 3 and a second layer 4. The first layer 3, which is illuminated by the laser beam, has properties similar to those of the recording layer of the information-carrying media used in ordinary magneto-optic discs, and also operates as a recording layer in the apparatus under discussion. The second layer 4, called the supplementary layer, is provided to enable overwriting; that is, to enable new data to be written over old data directly. Let Tc1 and Tc2 be the Curie temperatures of the first layer 3 and second layer 4 respectively, and let Hc1 and Hc2 be the coercive forces of first layer 3 and second layer 4. These parameters of the two layers satisfy the following relationships: EQU Tc1&lt;Tc2 EQU Hc1&gt;Hc2.
Let us first consider the reading of information recorded in the recording layer, namelythe first layer 3. The objective lens 5 is driven by a driving mechanism not shown in the drawings in the direction of its optic axis and in a direction perpendicular to its optic axis (the radial direction of the information-carrying medium) so as to keep the spot 6 focused on the recording layer and followed on an information-carrying track.
As shown in FIG. 5B, the recording layer is magnetized parallel to its thickness direction. The magnetic alignment is either up or down in the drawing, these two directions corresponding to the "0" and "1" of a binary code. Magnetization in the up direction in the drawing represents a binary "1". When the information is read, the spot 6 is focused onto the first layer 3. The magnetic alignment of the first layer 3 is converted to optical information through a well-known magneto-optic effect (such as the Kerr effect); thus the information on the information-carrying medium 1 is detected. The intensity of the laser beam on the information-carrying medium 1 in this read operation is equivalent to intensity a in FIG. 6. At this intensity the regions of the first layer 3 and second layer 4 illuminated by the focused spot 6 do not reach their Curie temperature Tc1 or Tc2, so the focused spot 6 does not erase the recorded information by destroying the magnetic alignment.
Information is overwritten as follows. The initializing magnet 8 in FIGS. 5A and 5B applies an external field Hini (indicated by arrow b in the drawing) to the information-carrying medium 1. The external field is related to the coercive forces of the first layer 3 and second layer 4 as follows: EQU Hini&lt;Hc1 EQU Hini&gt;Hc1.
As the information-carrying medium 1 rotates in direction a in FIG. 5B, when the second layer 4 passes over the initializing magnet 8 it is uniformly magnetized in the up direction, regardless of the magnetic alignment of the first layer 3. At room temperature the first layer 3 retains its existing magnetic alignment, being unaffected by the magnetic field generated by the initializing magnet 8 or the magnetic field generated by the magnetization of the second layer 4.
To write the information "1," that is, to magnetize the first layer 3 in the up direction, the laser beam is modulated to an intensity equivalent to b in FIG. 6. The temperature of the region of the first layer 3 where the spot 6 is focused by the objective lens 5 then rises until it exceeds the Curie temperature Tc1 of the first layer 3, destroying the magnetic alignment of the first layer 3. The second layer 4, however, remains below its Curie temperature Tc2, so it retains the upward magnetic alignment given it by the initializing magnet 8. When the portion of the first layer 3 illuminated by the focused spot 6 cools, it therefore acquires the upward magnetic alignment of the second layer 4.
To write the information "0," that is, to magnetize the first layer 3 in the down direction, the laser beam is modulated to an intensity equivalent to c in FIG. 6. In this case the temperature in the region illuminated by the focused spot 6 rises until it exceeds the Curie temperature in the first layer 3 (Tc1) and the second layer 4 (Tc2), causing both layers to lose their magnetic alignment. Due to a weak external magnetic field Hb generated by the bias magnet 9 located opposite the objective lens 5 on the other side of the information-carrying medium 1, however, the second layer 4 is remagnetized in the direction of the field Hb, namely the down direction. When the first layer 3 cools, it acquires the downward magnetic alignment of the second layer 4. In this way the first layer 3 is magnetized in the down direction. The intensity of the external bias field Hb is weak but within the range consistent with the above operation.
The operations described above enable new information to be overwritten in real time on old information by modulation of the laser beam between intensities b and c in FIG. 6, which write the binary codes "1" and "0".
When the magneto-optic information-carrying medium is structured as in the prior art described above, however, the initializing magnet must be located external to the housing of the information-carrying medium, in a space in the optical reading and writing apparatus. Consequently, the optical reading and writing apparatus must be complex in structure and large in size.